It is often desirable that an operational amplifier ("op amp") utilize a low power supply voltage. This severely constrains the voltage range of the common-mode part of the differential input signal supplied to many prior art bipolar op amps, particularly those whose input stage is a differential amplifier. In many cases, the common-mode voltage V.sub.CM of the input signal cannot use the entire range of the power supply voltage.
One way for extending the V.sub.CM range to the full supply range is to utilize complementary pairs of input transistors. This technique is used in the differential amplifier input section of the op amp in the Signetics NE660 integrated circuit. FIG. 1 shows the Signetics NE660 differential amplifier as, for example, disclosed in U.S. patent application Ser. No. 525,181, filed Aug. 23, 1983, now U.S. Pat. 4,532,479.
This amplifier has a differential input section containing complementary differential portions 10 and 12 which amplify the differential input signal defined by voltages V.sub.I+ and V.sub.I-. Voltage V.sub.I+ is V.sub.CM +.DELTA.V, and voltage V.sub.I- is V.sub.CM -.DELTA.V, where 2.DELTA.V is the differential part of the input signal. Current sources 14 and 16 respectively connected to points (or terminals) of low and high supply voltages V.sub.L and V.sub.H provide the supply current for the input section.
The input section is connected to a summing circuit 18 containing transistors QA, QB, QC, and QD configured as shown in FIG. 1. Equal-valued resistors RA, RB, RC, and RD are also part of circuit 18. As described more fully in the foregoing patent application, circuit 18 is a modulated current mirror for combining signal currents from differential portions 10 and 12 to provide an output current I.sub.O.
The range for the power supply voltage V.sub.PS --i.e., V.sub.H -V.sub.L --is divided into three ranges. A low range goes from V.sub.L up to slightly more than V.sub.L +V.sub.BE, where V.sub.BE is the absolute value of the standard base-to-emitter voltage of a bipolar transistor when it is just turned on. A high range goes from V.sub.H down to slightly less than V.sub.H -V.sub.BE. With V.sub.PS being greater than 2V.sub.BE, a middle range extends between the two end ranges.
Differential portion 10 contains NPN transistors Q1 and Q2 which amplify the input signal by dividing operating current provided from current source 14 on interconnected lines SA and SB into respective currents I.sub.A and I.sub.B transmitted on lines LA and LB. The difference between currents I.sub.A and I.sub.B is representative of the input signal when V.sub.CM is in the middle and high ranges where the operating current is at a constant supply level I.sub.L. As V.sub.CM drops into the low range, first one and then the other of transistors Q1 and Q2 turn off to shut down current source 14.
Similarly, differential portion 12 has PNP transistors Q3 and Q4 which divide operating current provided from current source 16 on interconnected lines SC and SD into respective currents I.sub.C and I.sub.D transmitted on lines LC and LD. The difference between currents I.sub.C and I.sub.D is representative of the input signal when V.sub.CM is in the low and middle ranges where the operating current for transistors Q3 and Q4 is at a constant supply level I.sub.H. They turn off to shut down current source 16 when V.sub.CM goes into the high range. Accordingly, portions 10 and 12 are both active in the middle range, but only one is active in each end range.
A measure of the amplification capability of a differential amplifier is its transconductance G.sub.MA (i.e., the ratio of the incremental change in total current through the input section to input voltage change .DELTA.V). Inasmuch as the individual transconductance of a bipolar transistor is approximately proportional to its collector current, G.sub.MA for FIG. 1 is approximately proportional to I.sub.A +I.sub.B +I.sub.C +I.sub.D. The total supply current transmitted by current sources 14 and 16 is the sum of the operating currents which, in turn, largely equals the sum of currents I.sub.A -I.sub.D. Since the current supply for the input section partly shuts down when V.sub.CM is in either end range, the total supply current is greater when V.sub.CM is in the middle range. The amplifier transconductance then varies in the same manner.
While the rail-to-rail input capability of the foregoing amplifier is advantageous, the G.sub.MA variation as V.sub.CM moves across the V.sub.PS range is sometimes a disadvantage. The variation in transconductance makes it difficult to optimize the frequency compensation for the amplifier when used in an op amp with negative feedback. Signal distortion occurs when one of current sources 14 and 16 shuts down. It would be desirable to have a differential amplifier arranged in a relatively simple configuration that achieves rail-to-rail input capability without G.sub.MA varying greatly across the supply range.